1. Field of the Invention
The present invention relates to a liquid-droplet jetting apparatus which jets a liquid droplet. For example, the present invention relates to a control for preventing an occurrence of a printing defect which is caused due to drying of a liquid near an opening of a nozzle, in a liquid-droplet jetting apparatus such as an ink-jet printer.
2. Description of the Related Art
A conventional liquid-droplet jetting apparatus such as an ink-jet printer includes a carriage on which a recording head having a plurality of nozzles discharging an ink is mounted. The recording head includes pressure chambers which communicate with the nozzles respectively, and an actuator which applies a pressure to the ink in the pressure chambers. An example of the actuator is a piezo-electric actuator. The ink is filled in the pressure chambers which communicate with these nozzles, and a drive pulse is applied to a piezoelectric actuator. Accordingly, a pressure deformable portion of the actuator is deformed, and a certain pressure chamber of the pressure chambers is expanded or contracted. Due to the expansion or contraction of the certain pressure chamber, a jetting pressure is applied to the ink in the certain pressure chamber. When the jetting pressure is applied to the ink, the ink is jetted on to a recording medium from a nozzle communicating with the certain pressure chamber, while the carriage performs a reciprocal moving.
Incidentally, in a recording head which performs a printing by jetting (discharging) the ink from the nozzles, when the printing is stopped, for a nozzle having a low frequency of jetting, a solvent (water etc.) in the ink is thickened due to gradual drying. Accordingly, a size of an ink droplet is reduced, and a malfunctioning such as the ink is hardly jetted occurs, which causes a decline in a printing performance. In such case, before a start of printing and/or during a printing operation, the carriage is moved periodically or forcibly up to a flushing portion which is a no-printing area, and by applying a drive pulse, a flushing operation (auxiliary jetting) of discharging the ink forcibly from each of the nozzles is performed, or, a so-called maintenance operation such as moving the carriage to a cap portion, and then performing a purge process of removing air bubbles and impurities by a forcible suction by applying a negative pressure to the nozzles, is performed.
However, while the purge process and the flushing process are effective in removing the air bubbles and impurities on one hand, the carriage is to be moved to the no-printing area other than the printing area, on the other hand. Therefore, there are problems such as a decrease in the printing speed, and a wasteful consumption of the ink. In Japanese Patent Application Laid-open No. H3-190747, and Japanese Patents No. 3318568 and 3613297, an arrangement is made such that when the ink is not jetted, apart from a drive pulse which jets the ink on to the recording medium, a no-jetting drive pulse which does not jet the ink is applied to the actuator, and by making a meniscus of the ink near the nozzle opening vibrate minutely, the drying of the ink is prevented to keep a viscosity of the ink low.
In Japanese Patent Application Laid-open No. H3-190747, particularly, an example in which, when the carriage is at the no-printing area, the meniscus is made to vibrate minutely in an effective manner, is described. An area in which the carriage is accelerated and decelerated and the recording head is in a no-printing state (acceleration and deceleration area, no-printing area), is provided on both sides of a recording area in which the printing is performed by causing the carriage mounted on the recording head to perform a reciprocating movement. An arrangement is made such that when the carriage moves from the printing area to this acceleration and deceleration area (no-printing area), minute vibrations are imparted to a meniscus of the ink by applying a voltage to the actuator, to an extent that an ink droplet is not jetted. Accordingly, the ink is supplied to a front end portion of the nozzles, and a uniform wetting is maintained in the front end portion of the nozzles. By making such an arrangement, upon printing of one line, in the no-printing area, minute vibrations are imparted to the front end portion of the nozzles, and the thickening of the ink in the front end portion of the nozzles is prevented.
A no-jetting drive pulse described in Japanese Patent No. 3318568, includes only a signal which is output with a frequency close to a natural frequency (characteristic frequency) of the pressure chamber when no recording is performed, and by applying such no-jetting drive pulse to the actuator, the pressure chamber is let to be in a resonance state. Applying the signal at the frequency close the natural frequency of the pressure chamber, it is possible to let the pressure chamber to be in the resonance state, even when the signal is applied for a short time, and it is possible to detach by vibration the air bubble from a wall surface of the pressure chamber. Therefore, (an operation of) applying this no-jetting drive pulse is effective. A time for which the signal of this frequency is applied may be a time of several cycles, and a several times to several tens of times of this time is let to be a stopping time (pause time), and reverberation of the vibration is converged (accumulated). The air bubbles or the impurities in the ink are removed effectively by repeating such series of no-jetting drives.
Moreover, in an ink-jet recording head described in U.S. Pat. No. 6,431,674 (corresponding to Japanese Patent No. 3613297), a first mode in which, at the time of stop which is the no-printing state, the no-jetting drive pulse (second driving signal) is applied intermittently to the actuator (piezoelectric resonator), is executed, and further, a second mode in which the no-jetting drive pulse is applied to the actuator for a time longer than the time in the first mode, is executed just before the printing. By changing the minute vibrations according to the mode, the thickened ink is stirred while reducing a fatigue of the actuator, and clogging of the nozzle is prevented.